U.S. patent application number 10/445130 was filed with the patent office on 2004-04-29 for gallium nitride based diodes with low forward voltage and low reverse current operation.
This patent application is currently assigned to Cree Lighting Company. Invention is credited to Mishra, Umesh, Parikh, Primit.
Application Number | 20040080010 10/445130 |
Document ID | / |
Family ID | 25429819 |
Filed Date | 2004-04-29 |
United States Patent
Application |
20040080010 |
Kind Code |
A1 |
Parikh, Primit ; et
al. |
April 29, 2004 |
Gallium nitride based diodes with low forward voltage and low
reverse current operation
Abstract
New Group III based diodes are disclosed having a low on state
voltage (V.sub.f) and structures to keep reverse current
(I.sub.rev) relatively low. One embodiment of the invention is
Schottky barrier diode made from the GaN material system in which
the Fermi level (or surface potential) of is not pinned. The
barrier potential at the metal-to-semiconductor junction varies
depending on the type of metal used and using particular metals
lowers the diode's Schottky barrier potential and results in a
V.sub.f in the range of 0.1-0.3V. In another embodiment a trench
structure is formed on the Schottky diodes semiconductor material
to reduce reverse leakage current. and comprises a number of
parallel, equally spaced trenches with mesa regions between
adjacent trenches. A third embodiment of the invention provides a
GaN tunnel diode with a low V.sub.f resulting from the tunneling of
electrons through the barrier potential, instead of over it. This
embodiment can also have a trench structure to reduce reverse
leakage current.
Inventors: |
Parikh, Primit; (Goleta,
CA) ; Mishra, Umesh; (Santa Barbara, CA) |
Correspondence
Address: |
KOPPEL, JACOBS, PATRICK & HEYBL
AN ASSOCIATION OF PROFESSIONAL LAW CORPORATIONS
SUITE 107
555 ST. CHARLES DRIVE,
THOUSAND OAKS
CA
91360
US
|
Assignee: |
Cree Lighting Company
|
Family ID: |
25429819 |
Appl. No.: |
10/445130 |
Filed: |
May 20, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10445130 |
May 20, 2003 |
|
|
|
09911155 |
Jul 23, 2001 |
|
|
|
Current U.S.
Class: |
257/471 ;
257/E29.338; 257/E29.339 |
Current CPC
Class: |
H01L 29/475 20130101;
H01L 29/88 20130101; H01L 29/2003 20130101; H01L 29/872 20130101;
H01L 29/8725 20130101 |
Class at
Publication: |
257/471 |
International
Class: |
H01L 027/095 |
Claims
We claim:
1. A group III nitride based diode, comprising: an n+ doped GaN
layer; an n- doped GaN layer on said n+ GaN layer; a Schottky metal
layer on said n- doped GaN layer having a work function, said n-
GaN layer forming a junction with said Schottky metal, said
junction having a barrier potential energy level that is dependent
upon the work function of said Schottky metal.
2. The diode of claim 1, wherein said barrier potential varies
directly with said Schottky metal work function.
3. The diode of claim 1, wherein said n- doped GaN layer has an
electron affinity, said barrier potential being generally equal to
said Schottky metal work function minus said electron affinity.
4. The diode of claim 1, further comprising a substrate adjacent to
said n+ GaN layer, opposite said n- doped GaN layer.
5. The diode of claim 4, wherein said substrate is sapphire
(Al.sub.2O.sub.3), silicon carbide (SiC) or silicon (Si).
6. The diode of claim 1, wherein said Schottky metal is one of the
metals from the group comprising Ti, Cr, Nb, Sn, W, Ta and Ge.
7. The diode of claim 1, wherein said n+ doped GaN layer is doped
with impurities to a concentration of at least 10.sup.18 per
centimeter cubed (cm.sup.3).
8. The diode of claim 1, wherein the n- doped GaN layer is doped
with impurities to a concentration in the range of
5.times.10.sup.14 to 5.times.10.sup.17 per cm.sup.3.
9. The diode of claim 1, further comprising a trench structure in
said n- doped GaN layer, said diode experiencing a reverse leakage
current under reverse bias, said trench structure reducing said
reverse leakage current.
10. The diode of claim 9, wherein said trench structure comprises a
plurality of trenches with mesa regions between adjacent trenches,
said trenches having sidewalls and a bottom surface coated by an
insulating material, said Schottky metal layer covering said
trenches and mesa regions, said insulating material sandwiched
between said Schottky metal layer and said sidewalls and bottom
surfaces.
11. The diode of claim 10, wherein said plurality of trenches are
parallel and equally spaced.
12. The diode of claim 10, wherein said insulating material is
SiN.
13. The diode of claim 10, wherein said insulating material is
replaced by a metal with a high work function.
14. The diode of claim 1, further comprising an ohmic contact on
said n+ GaN layer, a signal applied to said device across said
ohmic contact and said Schottky metal layer.
15. A diode, comprising: a layer of highly doped semiconductor
material having an unpinned surface potential; a layer of lower
doped semiconductor material adjacent to the highly doped
semiconductor material; and a Schottky metal layer on said lower
doped semiconductor material, said lower doped semiconductor
material forming a junction with said Schottky metal having a
barrier potential energy level that is dependent upon the type of
Schottky metal.
16. The diode of claim 15, wherein said doped layers are doped n
type.
17. The diode of claim 15, wherein said semiconductor material is a
Group III nitride.
18. The diode of claim 15, wherein said highly doped semiconductor
is n+ doped GaN layer and said lower doped semiconductor is n-
doped GaN layer.
19. The diode of claim 15, wherein said Schottky metal contact has
a work function, said barrier potential having an energy level that
varies directly with the work function of said Schottky metal.
20. The diode of claim 15, further comprising a substrate adjacent
to said n+ doped GaN layer, opposite said n- doped GaN layer.
21. The diode of claim 20, wherein said substrate is sapphire
(Al.sub.2O.sub.3), silicon carbide (SiC) or silicon (Si).
22. The diode of claim 15, wherein said Schottky metal is one of
the metals in the group comprising Ti, Cr, Nb, Sn, W, Ge and
Ta.
23. The diode of claim 18, wherein said n+ doped GaN layer is doped
with impurities to a concentration of at least 10.sup.18 per
centimeter cubed (cm.sup.3).
24. The diode of claim 18, wherein the n- doped GaN layer is doped
with impurities to a concentration in the range of
5.times.10.sup.14 to 5.times.10.sup.17 per cm.sup.3.
25. The diode of claim 15, further comprising a trench structure on
the surface of said lower doped semiconductor material, said diode
experiencing a reverse leakage current under reverse bias, said
trench structure reducing the amount of reverse leakage
current.
26. The diode of claim 25, wherein said trench structure comprises
a plurality of trenches with mesa regions between adjacent
trenches, each of said trenches having sidewalls and a bottom
surface coated by an insulating material, said Schottky metal layer
covering said trenches and mesa regions, said insulating material
sandwiched between said Schottky metal layer and said sidewalls and
bottom surfaces.
27. The diode of claim 26, wherein said insulating material is
replaced by a metal with a high work function.
28. The diode of claim 15, further comprising an ohmic contact on
said higher doped semiconductor material.
29. A tunneling diode comprising: an n+ doped layer; an n- doped
layer adjacent to said n+ doped layer; a barrier layer adjacent to
said n- doped layer, opposite said n+ layer; and a metal layer on
said barrier layer, opposite said n-doped layer, said n- doped
layer forming a junction with said barrier layer that has a barrier
potential which causes said diode's on state voltage to be low as a
result of electron tunneling through the barrier potential under
forward bias.
30. The diode of claim 29, wherein said barrier layer has
piezoelectric dipoles that lower the diode's on state voltage by
enhancing electron tunneling.
31. The diode of claim 29, wherein the number of piezoelectric
dipoles increases as the thickness of said barrier layer increases,
while still allowing tunneling currents.
32. The diode of claim 29, further comprising a substrate adjacent
to said n+ doped layer opposite said n- doped layer, said substrate
comprising sapphire, silicon carbide or silicon.
33. The diode of claim 29, wherein said n+ doped layer, n- doped
layer and barrier layer comprise polar materials.
34. The diode of claim 29, wherein said n+ doped layer, n- doped
layer and barrier layer are from the Group III nitride material
system.
35. The diode of claim 29, wherein said n+ doped layer is GaN, said
n- doped layer is GaN, and said barrier layer is AlGaN.
36. The diode of claim 29, wherein said n+ doped layer, n-doped
layer and barrier layer are formed from polar or non-polar
materials, or combinations thereof.
37. The diode of claim 29, wherein said n+ doped layer, n-doped
layer and barrier layer are formed from complex polar oxides such
as strontium titanate, lithium niobate, lead zirconium titanate, or
combinations thereof.
38. The diode of claim 29, wherein said n+ doped layer, n-doped
layer and barrier layer from binary polar oxides such as zinc
oxide.
39. The diode of claim 29, further comprising a trench structure in
said barrier and n- doped layers, said diode experiencing a reverse
leakage current under reverse bias, said trench structure reducing
the amount of said reverse leakage current.
40. The diode of claim 29, wherein said trench structure comprises
a plurality of trenches in said barrier and said n- layers having
mesa regions between adjacent trenches, each of said trenches
having sidewalls and a bottom surface coated by an insulating
material, said Schottky metal layer covering said trenches and mesa
regions, said insulating material sandwiched between said Schottky
metal layer and said sidewalls and bottom surfaces.
41. The diode of claim 40, wherein said insulating material is
replaced by a metal with a high work function.
42. The diode of claim 29, further comprising an ohmic contact on
said n+ doped layer.
43. A Schottky diode, comprising: a semiconductor material having
an unpinned surface potential; and a Schottky metal having a work
function and forming a junction with said semiconductor material
that has a barrier potential, the height of said barrier potential
depending upon said work function.
44. The diode of claim 43, wherein said semiconductor material is
Group III nitride based.
45. The diode of claim 43, wherein said semiconductor layer
comprises adjacent n- doped GaN and n+ doped GaN layers.
46. The diode of claim 45, further comprising an ohmic contact on
said n+ doped GaN layer, with said Schottky metal contacting said
n- GaN layer.
47. The diode of claim 43, wherein the height of said barrier
potential varies positively with the work function of said Schottky
metal.
48. The diode of claim 45, further comprising a substrate made of
sapphire (Al.sub.2O.sub.3), silicon carbide (SiC) or silicon (Si),
adjacent to the said n+ GaN layer, opposite said n- GaN layer.
49. The diode of claim 43, wherein said Schottky metal is one of
the metals in the group comprising Ti, Cr, Nb, Sn, W, Ta, Ge and
other metals with similar work functions.
50. The diode of claim 43, further comprising a trench structure in
said semiconductor material, said diode experiencing a reverse
leakage current under reverse bias, said trench structure reducing
said reverse leakage current.
51. The diode of claim 43, wherein said trench structure comprises
a plurality of trenches with mesa regions between adjacent
trenches, said trenches having sidewalls and a bottom surface
coated by an insulating material, said Schottky metal layer
covering said trenches and mesa regions, said insulating material
sandwiched between said Schottky metal layer and said sidewalls and
bottom surfaces.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to diodes, and more particularly to
gallium nitride (GaN) based diodes exhibiting improved forward
voltage and reverse leakage current characteristics.
[0003] 2. Description of the Related Art
[0004] Diode rectifiers are one of the most widely used devices for
low voltage switching, power supplies, power converters and related
applications. For efficient operation it is desirable for diodes to
have low on-state voltage (0.1-0.2V or lower), low reverse leakage
current, high voltage blocking capability (20-30V), and high
switching speed.
[0005] The most common diodes are pn-junction diodes made from
silicon (Si) with impurity elements introduced to modify, in a
controlled manner, the diode's operating characteristics. Diodes
can also be formed from other semiconductor materials such as
Gallium Arsenide (GaAs) and silicon carbide (SiC). One disadvantage
of junction diodes is that during forward conduction the power loss
in the diode can become excessive for large current flow.
[0006] Schottky barrier diodes are a special form of diode
rectifier that consist of a rectifying metal-to-semiconductor
barrier area instead of a pn junction. When the metal contacts the
semiconductor a barrier region is developed at the junction between
the two. When properly fabricated the barrier region will minimize
charge storage effects and improve the diode switching by
shortening the turn-off time. [L. P. Hunter, Physics of
Semiconductor Materials, Devices, and Circuits, Semiconductor
Devices, Page 1-10 (1970)] Common Schottky diodes have a lower
turn-on voltage (approximately 0.5V) than pn-junction diodes and
are more desirable in applications where the energy losses in the
diodes can have a significant system impact (such as output
rectifiers in switching power supplies).
[0007] One way to reduce the on-state voltage below 0.5V in
conventional Schottky diodes is to reduce their surface barrier
potential. This, however, results in a trade-off of increased
reverse leakage current. In addition, the reduced barrier can
degrade high temperature operation and result in soft breakdown
characteristics under reverse bias operation.
[0008] Also, Schottky diodes are commonly made of GaAs and one
disadvantage of this material is that the Fermi level (or surface
potential) is fixed or pinned at approximately 0.7 volts. As a
result, the on-state forward voltage (V.sub.f) is fixed. Regardless
of the type of metal used to contact the semiconductor, the surface
potential cannot be lowered to lower the V.sub.f.
[0009] More recently, silicon based Schottky rectifier diodes have
been developed with a somewhat lower V.sub.f. [IXYS Corporation, Si
Based Power Schottky Rectifier, Part Number DSS 20-0015B;
International Rectifier, Si Based Shottky Rectifier, Part Number
11DQ09]. The Shottky barrier surface potential of these devices is
approximately 0.4V with the lower limit of V.sub.f being
approximately 0.3-0.4 volts. For practical purposes the lowest
achievable Shottky barrier potential is around 0.4 volts with
regular metalization using titanium. This results in a V.sub.f of
approximately 0.25V with a current density of 100 A/cm.sup.2.
[0010] Other hybrid structures have been reported with a V.sub.f of
approximately 0.25V (with a barrier height of 0.58V) with operating
current density of 100 A/cm.sup.2. [M. Mehrotra, B. J. Baliga, "The
Trench MOS Barrier Shottky (TMBS) Rectifier", International
Electron Device Meeting, 1993]. One such design is the junction
barrier controlled Schottky rectifier having a pn-junction used to
tailor the electric fields to minimize reverse leakage. Another
device is the trench MOS barrier rectifier in which a trench and a
MOS barrier action are used to tailor the electrical field
profiles. One disadvantage of this device is the introduction of a
capacitance by the pn-junction. Also, pn-junctions are somewhat
difficult to fabricate in Group III nitride based devices.
[0011] The Gallium nitride (GaN) material system has been used in
opto-electronic devices such as high efficiency blue and green LEDs
and lasers, and electronic devices such as high power microwave
transistors. GaN has a 3.4 eV wide direct bandgap, high electron
velocity (2.times.10.sup.7 cm/s), high breakdown fields
(2.times.10.sup.6 V/cm) and the availability of
heterostructures.
SUMMARY OF THE INVENTION
[0012] The present invention provides new Group III nitride based
diodes having a low V.sub.f. Embodiments of the new diode also
include structures to keep reverse current (I.sub.rev) relatively
low.
[0013] The new diode is preferably formed of the GaN material
system, and unlike conventional diodes made from materials such as
GaAs, the Fermi level (or surface potential) of GaN is not pinned
at its surface states. In GaN Schottky diodes the barrier height at
the metal-to-semiconductor junction varies depending on the type of
metal used. Using particular metals will lower the diode's Schottky
barrier height and result in a V.sub.f in the range of
0.1-0.3V.
[0014] The new GaN Schottky diode generally includes an n+ GaN
layer on a substrate, and an n- GaN layer on the n+ GaN layer
opposite the substrate. Ohmic metal contacts are included on the n+
GaN layer, isolated from the n- GaN layer, and a Schottky metal
layer is included on the n- GaN layer. The signal to be rectified
is applied to the diode across the Schottky metal and ohmic metal
contacts. When the Schottky metal is deposited on the n- GaN layer,
a barrier potential forms at the surface of said n- GaN between the
two. The Schottky metal layer has a work function, which determines
the height of the barrier potential.
[0015] Using a metal that reduces the Schottky barrier potential
results in a low V.sub.f, but can also result in an undesirable
increase in I.sub.rev. A second embodiment of the present invention
reduces I.sub.rev by including a trench structure on the diode's
surface. This structure prevents an increase in the electric field
when the new diode is under reverse bias. As a result, the Schottky
barrier potential is lowered, which helps reduce I.sub.rev.
[0016] The trench structure is preferably formed on the n- GaN
layer, and comprises a number of parallel, equally spaced trenches
with mesa regions between adjacent trenches. Each trench has an
insulating layer on its sidewalls and bottom surface. A continuous
Schottky metal layer is on the trench structure, covering the
insulating layer and the mesas between the trenches. Alternatively,
the sidewalls and bottom surface of each trench can be covered with
metal instead of an insulator, with the metal electrically isolated
from the Schottky metal. The mesa regions have a doping
concentration and width chosen to produce the desired
redistribution of electrical field under the metal-semiconductor
contact.
[0017] A third embodiment of the invention provides a GaN tunnel
diode with a low V.sub.f resulting from the tunneling of electrons
through the barrier potential, instead of over it. This embodiment
has a substrate with an n+ GaN layer sandwiched between the
substrate and an n- GaN layer. An AlGaN barrier layer is included
on the n- GaN layer opposite the n+ GaN layer. An Ohmic contact is
included on the n+ GaN layer and a top contact is included on the
AlGaN layer. The signal to be rectified is applied across the Ohmic
and top contacts.
[0018] The barrier layer design maximizes the forward tunneling
probability while the different thickness and Al mole fraction of
the barrier layer result in different forward and reverse operating
characteristics. At a particular thickness and Al mole fraction,
the diode has a low V.sub.f and low I.sub.rev. Using a thicker
barrier layer and/or increasing the Al mole concentration decreases
V.sub.f and increases I.sub.rev. As the thickness or mole fraction
is increased further, the new diode will assume ohmic operating
characteristics, or become a conventional Schottky diode.
[0019] These and other further features and advantages of the
invention would be apparent to those skilled in the art from the
following detailed description, taking together with the
accompanying drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a sectional view of a GaN Schottky diode
embodiment of the invention;
[0021] FIG. 2 is a diagram showing the work function of common
metals verses their atomic number;
[0022] FIG. 3 is a band diagram for the diode shown in FIG. 1;
[0023] FIG. 4 is a sectional view of another embodiment of the GaN
Schotty diode of FIG. 1, having a trench structure to reduce
reverse current leakage;
[0024] FIG. 5 is a sectional view of a tunnel diode embodiment of
the invention;
[0025] FIG. 6 is a band diagram for the tunnel diode of FIG. 5
having a barrier layer with a thickness of 22 .ANG. and 30% Al mole
fraction;
[0026] FIG. 7 is a diagram showing the voltage/current
characteristics of the new tunnel diode having the band diagram of
FIG. 6;
[0027] FIG. 8 is a band diagram for the tunnel diode of FIG. 5
having a barrier layer with a thickness of 30 .ANG. and 30% Al mole
fraction;
[0028] FIG. 9 is a diagram showing the voltage/current
characteristics of the new tunnel diode having the band diagram of
FIG. 8;
[0029] FIG. 10 is a band diagram for the tunnel diode of FIG. 5
having a barrier layer with a thickness of 38 .ANG. and 30% Al mole
fraction;
[0030] FIG. 11 is a diagram showing the voltage/current
characteristics of the new tunnel diode having the band diagram of
FIG. 10; and
[0031] FIG. 12 is a sectional view of a tunnel diode embodiment of
the invention having a trench structure to reduce reverse current
leakage.
DETAILED DESCRIPTION OF THE INVENTION
[0032] FIG. 1 shows a Schottky diode 10 constructed in accordance
with the present invention having a reduced metal-to-semiconductor
barrier potential. The new diode is formed of the Group III nitride
based material system or other material systems where the Fermi
level is not pinned at its surface states. Group III nitrides refer
to those semiconductor compounds formed between nitrogen and the
elements in Group III of the periodic table, usually aluminum (Al),
gallium (Ga), and indium (In). The term also refers to ternary and
tertiary compounds such as AlGaN and AlInGaN. The preferred
materials for the new diode are GaN and AlGaN.
[0033] The new diode 10 comprises a substrate 11 that can be either
sapphire (Al.sub.2O.sub.3), silicon (Si) or silicon carbide (SiC),
with the preferred substrate being a 4H polytype of silicon
carbide. Other silicon carbide polytypes can also be used including
3C, 6H and 15R polytypes. An Al.sub.xGa.sub.1-xN buffer layer 12
(where x in between 0 and 1) is included on the substrate 11 and
provides an appropriate crystal structure transition between the
silicon carbide substrate and the remainder of the diode 10.
[0034] Silicon carbide has a much closer crystal lattice match to
Group III nitrides than sapphire and results in Group III nitride
films of higher quality. Silicon carbide also has a very high
thermal conductivity so that the total output power of Group III
nitride devices on silicon carbide is not limited by the thermal
dissipation of the substrate (as is the case with some devices
formed on sapphire). Also, the availability of silicon carbide
substrates provides the capacity for device isolation and reduced
parasitic capacitance that make commercial devices possible. SiC
substrates are available from Cree Research, Inc., of Durham, N.C.
and methods for producing them are set forth in the scientific
literature as well as in a U.S. Pat. Nos. Re. 34,861; 4,946,547;
and 5,200,022.
[0035] The new diode 10 has an n+ GaN layer 12 on a substrate 11
and an n- layer of GaN 13 on the n+ GaN layer 12, opposite the
substrate 11. The n+ layer 12 is highly doped with impurities to a
concentration of at least 10.sup.18 per centimeter cubed
(cm.sup.3), with the preferable concentration being 5 to 10 times
this amount. The n- layer 13 has a lower doping concentration but
is still n-type and it preferably has an impurity concentration in
the range of 5.times.10.sup.14 to 5.times.10.sup.17 per cm.sup.3.
The n- layer 13 is preferably 0.5-1 micron thick and the n+ layer
12 is 0.1 to 1.5 microns thick, although other thicknesses will
also work.
[0036] Portions of the n- GaN layer 13 are etched down to the n+
layer and ohmic metal contacts 14a and 14b are included on the n+
GaN layer in the etched areas so that they are electrically
isolated from the n- GaN layer 13. In an alternative embodiment,
one or more ohmic contacts can be included on the surface of the
substrate that is not covered by the n+ GaN layer 12. This
embodiment is particularly applicable to substrates that are
n-type. A Schottky metal layer 16 is included on the n- GaN layer
13, opposite the n+ GaN layer 12.
[0037] The work function of a metal is the energy needed to take an
electron out of the metal in a vacuum and the Fermi level of a
material is the energy level at which there is a 50% probability of
finding a charged carrier. A semiconductor's electron affinity is
the difference between its vacuum energy level and the conduction
band energy level.
[0038] As described above, the surface Fermi level of GaN is
unpinned and as a result, Schottky metals with different work
functions result in different barrier potentials. The barrier
potential is approximated by the equation:
Barrier Height=work function-the semiconductor's electron
affinity
[0039] FIG. 2 is a graph 20 showing the metal work function 21 for
various metal surfaces in a vacuum, verses the particular metal's
atomic number 22. The metal should be chosen to provide a low
Schottky barrier potential and low V.sub.f, but high enough so that
the reverse current remains low. For example, if a metal were
chosen having a work function equal to the semiconductor's electron
affinity, the barrier potential approaches zero. This results in a
V.sub.f that approaches zero and also increases the diode's reverse
current such that the diode becomes ohmic in nature and provides no
rectification.
[0040] Many different metals can be used to achieve a low barrier
height, with the preferred metals including Ti(4.6 work function)
23, Cr(4.7) 24, Nb(4.3) 25, Sn(4.4) 26, W(4.6) 27 and Ta (4.3) 28.
Cr 24 results in an acceptable barrier potential and is easy to
deposit by conventional methods.
[0041] FIG. 3 shows a typical band diagram 30 for the new Schottky
barrier diode taken on a vertical line through the diode. It shows
the energy levels of Schottky metal 31, the GaN semiconductor
layers 32, and the Shottky barrier potential 33.
[0042] Prior to contact of the GaN semiconductor material by the
Schottky metal, the Fermi energy levels of the two are not the
same. Once the contact is made and the two materials become a
single thermodynamic system, a single Fermi level for the system
results. This is accomplished by the flow of electrons from the
semiconductor material, which has a higher Fermi level, to the
Schottky metal, which has a lower Fermi level. The electrons of the
semiconductor lower their energy by flowing into the metal. This
leaves the ionized donor levels of the semiconductor somewhat in
excess of the number of its free electrons and the semiconductor
will have a net positive charge. Electrons that have flowed from
the semiconductor into the metal cause the metal have a negative
electrostatic charge. The energy levels of the semiconductor are
accordingly depressed, and those of the metal are raised. The
presence of this surface charge of electrons and the presence of
unneutralized charge ionized donor levels of the semiconductor
create the dipole layer which forms the barrier potential.
[0043] In operation, the signal to be rectified by the new Schottky
diode 10 is applied across the Schottky metal 14 and the ohmic
contacts 14a and 14b. The rectification of the signal results from
the presence of the barrier potential at the surface of the n- GaN
layer 13, which inhibits the flow of charged particles within the
semiconductor. When the Schottky metal 16 is positive with respect
to the semiconductor (forward bias), the energy at the
semiconductor side of the barrier is raised. A larger number of
free electrons on the conduction band are then able to flow into
the metal. The higher the semiconductor side is raised, the more
electrons there are at an energy above the top of the barrier,
until finally, with large bias voltages the entire distribution of
free electrons in the semiconductor is able to surmount the
barrier. The voltage verses current characteristics become Ohmic in
nature. The lower the barrier the lower the V.sub.f necessary to
surmount the barrier.
[0044] However, as discussed above, lowering the barrier level can
also increase the reverse leakage current. When the semiconductor
is made positive with respect to the metal (reverse bias), the
semiconductor side of the barrier is lowered relative to the metal
side so that the electrons are free to flow over the top of the
barrier to the semiconductor unopposed. The number of electrons
present in the metal above the top of the barrier is generally very
small compared to the total number of electrons in the
semiconductor. The result is a very low current characteristic.
When the voltage is large enough to cut-off all flow of electrons,
the current will saturate. The lower the barrier potential, the
smaller reverse biases needed for the current to saturate.
[0045] FIG. 4 shows another embodiment of the new GaN Schottky
diode 40 that addresses the problem of increased reverse current
with decreased barrier height. The diode 40 is similar to the above
embodiment having a similar substrate 41, n+ GaN layer 42, and
Ohmic metal contacts 43a and 43b, that can alternatively be
included on the surface of the substrate. It also has an n- GaN
layer 44, but instead of this layer being planar, it has a two
dimensional trench structure 45 that includes trenches 46 in the
n-GaN layer. The preferred trench structure 45 includes trenches 46
that are parallel and equally spaced with mesa regions 49 remaining
between adjacent trenches. Each trench 46 has an insulating layer
47 covering its sidewalls 46a and bottom surface 46b. Many
different insulating materials can be used with the preferred
material being silicon nitride (SiN). A Schottky metal layer 48 is
included over the entire trench structure 45, sandwiching the
insulating layer between the Schottky metal and the trench
sidewalls and bottom surface, and covering the mesa regions 49. The
mesa regions provide the direct contact area between the Schottky
metal and the n- GaN layer 44. Alternatively, each trench can be
covered by a metal instead of an insulator. In this embodiment, the
Schottky metal should be insulated and/or separated from the trench
metal.
[0046] The mesa region 49 has a doping concentration and width
chosen to produce a redistribution of electrical field under the
mesa's metal-semiconductor junction. This results in the peak of
the diodes electrical field being pushed away from the Schottky
barrier and reduced in magnitude. This reduces the barrier lowering
with increased reverse bias voltage, which helps prevent reverse
leakage current from increasing rapidly.
[0047] This redistribution occurs due to the coupling of the charge
in the mesa 49 with the Schottky metal 48 on the top surface and
with the metal on the trench sidewalls 46a and bottom surface 46b.
The depletion then extends from both the top surface (as in a
conventional Schottky rectifier) and the trench sidewalls 46a,
depleting the conduction area from the sidewalls. The sidewall
depletion reduces the electrical field under the Schottky metal
layer 48 and can also be thought of as "pinching off" the reverse
leakage current. The trench structure 45 keeps the reverse leakage
current relatively low, even with a low barrier potentials and a
low V.sub.f.
[0048] The preferred trench structure 45 has trenches 46 that are
one to two times the width of the Schottky barrier area.
Accordingly, if the barrier area is 0.7 to 1.0 microns, the trench
width could be in the range of 0.7 to 2 microns.
[0049] The above diodes 10 and 40 are fabricated using known
techniques. Their n+ and n- GaN layers are deposited on the
substrate by known deposition techniques including but not limited
to metal-organic chemical vapor deposition (MOCVD). For diode 10,
the n- GaN layer 13 is etched to the n+ GaN layer 12 by known
etching techniques such as chemical, reactive ion etching (RIE), or
ion mill etching. The Schottky and Ohmic metal layers 14, 14b and
16 are formed on the diode 10 by standard metallization
techniques.
[0050] For diode 40, after the n+ and n- layers 42 and 44 are
deposited on the substrate, the n- GaN layer 44 is etched by
chemical or ion mill etching to form the trenches 46. The n- GaN
layer 44 is further etched to the n+ GaN layer 42 for the ohmic
metal 43a and 43b. The SiN insulation layer 47 is then deposited
over the entire trench structure 45 and the SiN layer is etched off
the mesas 49. As a final step, a continuous Schottky metal layer 48
is formed by standard metalization techniques over the trench
structure 45, covering the insulation layers 47 and the exposed
trench mesas 49. The ohmic metal is also formed on the n+ GaN layer
42 by standard metalization techniques. In the embodiments of the
trench diode where the trenches are covered by a metal, the metal
can also be deposited by standard metalization techniques.
[0051] Tunnel Diode
[0052] FIG. 5 shows another embodiment 50 of the new diode wherein
V.sub.f is low as a result of electron tunneling through the
barrier region under forward bias. By tunneling through the barrier
electrons do not need to cross the barrier by conventional
thermionic emission over the barrier.
[0053] Like the embodiments in FIGS. 1 and 4, the new tunnel diode
50 is formed from the Group III nitride based material system and
is preferably formed of GaN, AlGaN or InGaN, however other material
systems will also work. Combinations of polar and non-polar
materials can be used including polar on polar and polar on
non-polar materials. Some examples of these materials include
complex polar oxides such as strontium titanate, lithium niobate,
lead zirconium titanate, and non-complex/binary oxides such as zinc
oxide. The materials can be used on silicon or any
silicon/dielectric stack as long as tunneling currents are
allowed.
[0054] The diode 50 has a substrate 51 comprised of either
sapphire, silicon carbide (SiC) or silicon Si, with SiC being the
preferred substrate material for the reasons outlined above. The
substrate has an n+ GaN layer 52 on it, with an n- GaN layer 53 on
the n+ GaN layer 52 opposite the substrate 51. An AlGaN barrier
layer 54 is included on the n- GaN layer opposite the n+ GaN
template layer 52. At the edges of the diode 50, the barrier layer
54 and n- GaN layer 53 are etched down to the n+ GaN layer 52 and
ohmic metal contacts 55a and 55b are included on the layer 52 in
the etched areas. As with the above structures, the ohmic contacts
can also be included on the surface of the substrate. A metal
contact layer 56 is included on the AlGaN barrier layer 54,
opposite the n-GaN layer 53. The signal to be rectified is applied
across the ohmic contacts 55a and 55b and top metal contact 56.
[0055] The AlGaN barrier layer 54 serves as a tunnel barrier.
Tunneling across barriers is a quantum mechanical phenomenon and
both the thickness and the Al mole fraction of the layer 54 can be
varied to maximize the forward tunneling probability. The AlGaN-GaN
material system a has built in piezoelectric stress, which results
in piezoelectric dipoles. Generally both the piezoelectric stress
and the induced charge increases with the barrier layer thickness.
In the forward bias, the electrons from the piezoelectric charge
enhance tunneling since they are available for conduction so that
the number of states from which tunneling can occur is increased.
Accordingly the new tunnel diode can be made of other polar
material exhibiting this type of piezoelectric charge.
[0056] However, under a reverse bias the piezoelectric charge also
allows an increase in the reverse leakage current. The thicker the
barrier layer or increased Al mole fraction, results in a lower
V.sub.f but also results in an increased I.sub.rev. Accordingly,
there is an optimum barrier layer thickness for a particular Al
mole fraction of the barrier layer to achieve operating
characteristics of low V.sub.f and relatively low I.sub.rev.
[0057] FIGS. 6-11 illustrate the new diode's rectification
characteristics for three different thicknesses of an AlGaN barrier
layer with 30% Al. For each thickness there is a band energy
diagram and a corresponding voltage vs. current graph.
[0058] FIG. 6 shows the band diagram 60 for the tunnel diode 50
having 22 .ANG. thick barrier layer 54. It shows a typical barrier
potential 61 at the junction between the barrier layer 63 and the
n- GaN semiconductor layer 62. The top contact metal 64 is on the
barrier layer 63, opposite the semiconductor layer. FIG. 7 shows a
graph 70 plotting the corresponding current vs. voltage
characteristics of the diode in FIG. 6. It has a V.sub.f 71 of
approximately 0.1V and low reverse current (I.sub.rev) 72.
[0059] FIG. 8 shows a band diagram 80 for the same tunnel diode
with a 30 .ANG. thick barrier layer. The increase in the barrier
layer thickness increases the barrier region's piezoelectric
charge, thereby enhancing tunneling across the barrier. This
flattens the barrier potential 81 at the junction between the
barrier layer 82 and the n-GaN layer 83. Charges do not need to
overcome the barrier when a forward bias is applied, greatly
reducing the diode's V.sub.f. However, the flattened barrier also
allows for increase reverse leakage current (I.sub.rev). FIG. 9 is
a graph 90 showing the V.sub.f 91 that is lower than the V.sub.f in
FIG. 7. Also, I.sub.rev 92 is increased compared to I.sub.rev in
FIG. 7.
[0060] FIG. 10 shows a band diagram 100 for the same tunnel diode
with a 38 .ANG. thick barrier layer. Again, the increase in the
barrier layer thickness increases the piezoelectric charge. At this
thickness, the barrier potential 101 between the barrier layer 102
and n- GaN layer tails down near the junction between the barrier
layer and n- GaN layer, which results in there being no barrier to
charges in both forward and reverse bias. FIG. 11 shows a graph 110
of the corresponding current vs. voltage characteristics. The diode
100 experiences immediate forward and reverse current in response
to forward and reverse bias such that the diode becomes ohmic in
nature.
[0061] In the case where the mole concentration of aluminum in the
barrier layer is different, the thicknesses of the layers would be
different to achieve the characteristics shown in FIGS. 6 through
11.
[0062] FIG. 12 shows the new tunneling diode 120 with a trench
structure 121 to reduce reverse current. Like the Schottky diode 40
above, the trench structure includes a number of parallel, equally
spaced trenches 122, but in this diode, they are etched through the
AlGaN barrier layer 123 and the n- GaN layer 124, to the n+ GaN
layer 125 (AP GaN Template). There are mesa regions 126 between
adjacent trenches 122. The trench sidewalls and bottom surface have
an insulation layer 127 with the top Schottky metal layer 128
covering the entire trench structure 121. The trench structure
functions in the same way as the embodiment above, reducing the
reverse current. This is useful for the tunnel diodes having
barrier layers of a thickness that results in immediate forward
current in response to forward voltage. By using trench structures,
the diode could also have improved reverse current leakage. Also
like above, the trench sidewalls and bottom surface can be covered
by a metal as long as it is isolated from the Schottky metal layer
128.
[0063] Although the present invention has been described in
considerable detail with reference to certain preferred
configurations thereof, other versions are possible. Therefore, the
spirit and scope of the appended claims should not be limited to
the preferred versions described in the specification.
* * * * *